Sustainable energy sources, for example wind turbines, are gaining widespread popularity due to increased energy demands and the desire to reduce consumption of natural resources.
A typical wind turbine comprises a plurality of rotor blades, located atop a high tower, for converting the wind energy to rotational energy for driving a main shaft. The main shaft is coupled to an electric generator either directly or through a gearbox (transmission). The gearbox converts low speed wind-driven rotation to high speed rotation as required for driving the generator to generate electricity. The wind turbine also includes a structural support component, such as a tower, and a rotor pointing mechanism.
Wind turbine control tends to be complex, as wind speeds fluctuate in both intensity and direction. Horizontal and vertical wind shears, mechanical oscillations, and yaw misalignment, together with natural wind turbulence and tower motion, also induce dynamic and asymmetric loads on the rotor blades. These loads are transferred to the rotating main turbine shaft where they appear as forces or bending/twisting/torque moments. Specifically, these loads generate large torques, bending moments, twisting moments, stress forces and strain forces. For a wind turbine, the shaft torque may also have dynamic components induced by current flowing on the electrical grid and the turbine control system. These dynamic components are also of interest from a design, control and reliability standpoint.
The forces imposed by these operating conditions, sometimes referred to as loads, also increase the number of fatigue cycles accumulated by the wind turbine. Such loads and fatigue cycles can lead to premature system failure, operational inefficiencies, and damage to the wind turbine components.
To ensure reliable and efficient operation, wind turbine control systems should accurately measure the forces and the bending/twisting/torque moments acting on the shaft and control one or more operational parameters of the wind turbine system, such as the blade pitch, revolutions per second and/or yaw angle, to limit these forces. Accurate measurement of rotational speed of the shaft and shaft position (i.e., an angle a fixed point on the shaft makes with a fixed point external to the shaft) are also required for proper and safe operation of the wind turbine. The accuracy of these measurements must be maintained over a relatively long period. Wind turbine control also becomes more complex as the wind turbine size and energy output increase. In addition to using these measured values to control the wind turbine, the measured values can be used in wind turbine design.
To address the design and operation of any equipment using a rotating shaft, it is desired to measure any external force-induced deformations at the shaft surface. These measurements can be used to numerically determine the bending/twisting/torque and moments and other forces imposed on the shaft.
Conventional shaft control technologies employ a number of different sensors and/or systems to sense or measure these forces and shaft operating parameters. These sensors include, but are not limited to, strain gauge systems, encoder/tooth systems, acoustic wave systems, elastic systems, magnetostrictive systems and magnetoelastic systems. Each of these systems has certain characteristics and applications, as well as specific advantages and disadvantages.
Strain gauges embedded in or attached to the shaft provide local shaft strain measurements. These gauges require an electrical coupling to the rotating shaft, i.e., a physical connection (e.g., slip rings) or a wireless connection, and the signals produced have a relatively low signal-to-noise ratio. The strain gauges also suffer from low stability, limited bandwidth and tend to require frequent calibration. The limited operating temperature range of strain gauges limits their use in harsh environments. Also, strain gauges may fail after a short period of use due to the large stresses imposed on the shaft in applications with large diameter shafts in high power applications. Thus strain gauges are seldom used in commercial power train equipment.
An encoder/tooth-wheel torque sensor requires some mechanical interaction with the rotating shaft, such as by a magnetic tooth-wheel. But the tooth-wheel design tends to be costly and impractical for many applications. Such a design is not practical for higher speed applications, imposes reliability issues in a harsh environment and although stable, lacks high resolution.
An acoustic wave system utilizes sensors, such as surface acoustic wave (SAW) and bulk acoustic wave (BAW) sensors, mounted on the shaft for measuring shaft strain. Slip rings or a wireless system are required to carry the signals indicating shaft deformations and forces imposed on the shaft to an external detector.
Elastic torque systems measure the twisting of the shaft by measuring angular displacement of markers disposed across a length of the shaft. This system may not be sufficiently accurate for large diameter shafts and may have practical implementation problems.
Proximity sensors are also employed to measure shaft bending moments. These sensors require a stiff reference (i.e., a stiff support structure) and are vulnerable to deflection of the support structure and sensor drift, leading to errors in measured values. Since the main shaft system is stiff, small offset errors in the measured, such as 0.1 mm, correspond to high errors in the bending moment analysis, such as an error of 200 kNm. These errors can cause improper operation of the shaft control system.
Shaft position can be determined by angular encoders that employ optical gratings. The shaft is encoded prior to installation and the encoded regions detected to determine shaft position. But these sensors are prone to contamination and failure in dirty environments.
Magnetic shaft force sensors, as described by NCT Engineering GmbH. (Erlenhof-Park. Inselkammerstr. 10, 82008 Unterhaching, Germany) and others, cannot be applied to large shafts in a cost efficient manner, e.g. on shaft diameters greater than about 200 mm, due to the high power required to encode the shaft.
Another approach to measuring forces imposed on the shaft is based on the magnetostrictive effect on ferromagnetic shaft material or on ferromagnetic material regions applied to or formed in the shaft. Magnetostrictive measurements are based on the phenomenon that a material changes dimensions when magnetized. For certain materials the magnetostrictive effect is very small.
A conventional magnetostrictive torque sensor comprises a primary coil that generates a high frequency magnetic field and secondary coils that measure the magnetic flux of the resulting field. The total measured flux from all of the secondary coils indicates whether a torque is present. This approach does not require encoding of the shaft.
Typical magnetostrictive coefficients, in the form Δl/l, are on the order of 1×10−6 to 25×106. The use of the direct (i.e., no encoding of the ferromagnetic material) magnetostrictive effect for measuring torque on large shafts of ferromagnetic material is expensive, requires complex sensor arrangements, difficult calibration procedures and typically results in measurements with limited accuracy.
However, the magnetostrictive effect can be advantageously used with improved accuracy and reduced installation costs by combining the magnetostrictive effect with a magnetically encoded shaft or with magnetically encoded regions applied to the shaft. The shaft material or the material regions are encoded by passing current through the shaft or material regions during shaft manufacture or after installation of the shaft. The encoding is permanent when applied to a suitable material and when created by a current with a sufficiently high current density.
Encoding electrodes are electrically coupled to the shaft to support current flow from one or more input electrodes through regions of the shaft to one or more output electrodes. The current induces a magnetic field that creates magnetically polarized encoded regions within the shaft. When the encoding current and the resultant encoding magnetic field are applied to a ferromagnetic material, the boundaries between magnetic domains shift and the domains rotate. Both of these effects change dimensions of the material along the magnetic axis. Preferably the encoding electrodes are disposed to create a plurality of uniform magnetic regions on the shaft.
Conversely, one or more magnetic parameters of the material change when subjected to a mechanical force or a bending/twisting/torque moment. Specifically, these forces change the material properties and in turn cause a change in an external component of the magnetic field. These changes in the magnetic field can be detected by magnetostrictive sensors, such as fluxgate sensors.
A typical magnetostrictive torque sensor employs total shaft encoding, with the magnetization created by axial current flow along the shaft. The encoding is circumferentially uniform (circumferentially uniform) as the magnetic encoding requires magnetization of the entire cross-section. To create these uniform circumferential magnetic regions, multiple electrodes are disposed in ring-like arrays around the shaft and current is simultaneously applied to all electrodes. The magnetization is created (i.e., the shaft is encoded) by directing current to flow in an axial direction along the shaft from input electrodes to output electrodes.
However, large diameter shafts, such as wind turbine shafts (and gas turbine shafts), are typically not amenable to the conventional magnetic encoding technique as described immediately above. These techniques are suitable for relatively small diameter shafts but as the shaft diameter increases, the number of electrodes required to magnetically encode the shaft increases and the required current carried by each electrode also increases. For example, a current of several hundred amperes may be required for each electrode pair (a pair comprising an input and an output electrode). For accurate torque detection (or detection of any forces exerted on the shaft), the encoding must create a circumferentially uniform magnetic field; a difficult and costly effort to implement on large diameter shafts. Disadvantageously, the rotational speed of the shaft cannot be determined from a circumferentially uniform magnetic field.
Non-uniform magnetic fields are caused by non-homogeneity of the electrical and magnetic properties of the shaft. Further, the current is typically supplied as specifically-shaped current pulses, requiring complex electronic circuits to support the high-current. For all of these reasons, circumferentially uniformly encoding schemes applied to large diameter shafts tend to be difficult and very expensive to implement.
Examples of prior art magnetostrictive encoding and sensing is described with reference to FIGS. 1-4. Referring to FIG. 1, a shaft 5 comprises a ferromagnetic material. Spaced-apart ring-like electrodes 10 and 15 are disposed about a shaft circumference to encode an axial region 20 between the electrodes 10 and 15. Both the electrodes 10 and 15 are in electrical contact with the shaft. Spacing the electrodes apart tends to promote a uniform magnetic flux density within the region 20, thereby creating a circumferentially uniform encoded region. Uniformity of the flux density also depends on several other factors, including the shaft diameter. Additional pairs of electrodes (not shown), such as the electrodes 10 and 15, are axially disposed along the shaft to encode additional regions for detecting forces imposed at other shaft segments.
During the encoding process a current pulse 25 is applied to the electrode 15 to establish a current flow 30 along the longitudinal axis of the shaft 5 and within the region 20. After flowing along the region 20, the current is received by the electrode 10 to produce an output current 35. Current flow through the encoded region 20 induces a magnetic field that aligns the magnetic domains. Permanent magnetization of the shaft regions requires a high current density within that region.
All magnetic field sensor techniques that employ permanent magnetization of the shaft, such as described above, detect the externally-measurable magnetic field caused by the permanent magnetization. These field sensors also detect changes in the magnetic field that are caused by bending/twisting/torque and other forces. These forces change the magnetic permeability of the material, thereby altering some aspect of the magnetic field in the material and also altering the external magnetic field. Depending on the geometry of the unaltered field and the nature of the imposed forces, the forces may change the direction of the field or the intensity of the field (i.e., either a change in the field intensity or the flux density) or both.
Generally it is common in the art to refer to an altered magnetic field as one that includes changes in field strength or magnetic flux. A distorted field typically refers to changes only in a direction of the magnetic field.
When the shaft 5 is in operation, sensor coils 45 (only one shown in FIG. 2) mounted proximate the rotating shaft 5 sense the magnetic field and produce a signal representative of that field. With no stress or torque applied, the sensors do not detect any magnetic field distortions or alterations. Such sensors typically exhibit a directionality characteristic, as uniaxial sensors cannot discriminate changes in direction and strength.
The sensor coils 45 comprise fluxgate sensors, or other magnetic field sensors such as coil sensors, inductive sensors, or Hall effect sensors.
When a torque is imposed on the shaft 5 or a region of the shaft 5, the altered magnetic field emerging from the encoded region 20 is detected by the sensor coils 45. The sensor coils 45 are typically coupled to electronic processing components for analyzing and displaying the magnetic field distortions and alterations, and for indicating the imposed forces, especially including torque.
The prior art system as described above and illustrated in FIGS. 1 and 2 employs axial current flow to create uniform circumferentially uniform shaft magnetization. This technique requires magnetization of the entire shaft circumference and is therefore not practical for larger diameter shafts, as these shafts require a large encoding current to produce sufficient flux densities to create permanent and uniform magnetic regions in the shaft. While technically feasible and possible, the requirement for these large currents makes it expensive to achieve a uniform current distribution and density in a circumferential direction for large diameter shafts. Thus this encoding scheme is typically limited to smaller shafts below approximately 200 mm in diameter.
To alleviate concerns associated with large diameter shafts and the attendant requirement for large currents, one known technique uses multiple electrical connections to the shaft 5 as shown in FIG. 3. Spaced-apart rings 50 and 55 are disposed proximate the shaft 5 and insulated from the shaft 5, with each ring 50/55 having multiple electrical conductors 60 that are attached to the shaft 5. An input current 65 supplied to the ring 50 flows through the conductors 60 then axially through the region 80 and emerges through the ring 55. Current flow through the region 80 produces multiple magnetized regions 75 (only one shown in FIG. 3).
The complex encoding arrangement of FIG. 3 requires a small spacing between the rings 50 and 55 relative to the shaft diameter. Otherwise, a sufficiently uniform magnetization in a circumferential direction is not achievable. Larger spacing between the rings 50 and 55 increases the length of the region 80, which causes implementation problems and additional expenses in many applications. In addition, individual currents applied to the electrical conductors 60 must all have the same amplitude, requiring precise control and considerable expense to implement in larger diameter shafts.
Co-owned patent application publication 2009/0301223 (application Ser. No. 12/134,689) describes and claims yet another encoding scheme for use with large diameter shafts. This patent application publication is incorporated herein by reference. FIG. 4 depicts a shaft 205 having magnetically polarized encoded regions or channels formed by an encoding structure 210. A material of the shaft 205 comprises a ferromagnetic material or a ferromagnetic material affixed to the shaft 205. Alternating conducting members 215 and 217 are axially-positioned along a portion of the shaft 205 and supported by a non-conductive frame 212. The members 215 and 217 are disposed proximate the shaft 205 with a gap between each member 215 and 217 and a surface of the shaft 205. The positive encoding conducting members 215 alternate with negative encoding conducting members 217.
A first end of each conducting member 215 is coupled to a positive terminal of an encoding or current source 250 (only one illustrated in FIG. 4) and a second end is coupled to the shaft 205 at an electrode 218 via a conductor 242. A negative terminal of the encoding source 250 is coupled to an electrode 247 disposed on the shaft 205.
A first end of each conducting member 217 is coupled to a negative terminal of an encoding or current source 252 (only one illustrated in FIG. 4) and a second end is coupled to the shaft 205 at an electrode 220 via a conductor 243. A positive terminal of the encoding source 252 is coupled to an electrode 248 disposed on the shaft 205.
Electrical current from each conducting member 215 travels through the shaft 205 in a direction as indicated along a path 245 to generate a positive magnetically polarized channel 260 (only one shown in FIG. 4) on the shaft 205. Similarly, electrical signals from each conducting member 217 travel through the shaft 205 in a direction as indicated along a path 249 to generate a negative magnetically polarized channel 262 (only one shown in FIG. 4) on the shaft 205. The direction of current flow for the paths 245 and 249 are in opposite directions and thus the magnetic domains are oppositely polarized (positive or negative) within the magnetized channels 260 and 262.
When the shaft 205 is in operation, magnetic fields produced by the positive and negative magnetically polarized channels 260 and 262 have an expected shape and are detected by sensors (not shown in FIG. 4). When a torque or another force acts on the shaft 205, the magnetic fields produced by the channels 260 and 262 are altered or distorted. The sensors detect these changes and responsive thereto indicate the presence of a force within the encoded regions (i.e., the region including the channels 260 and 262) of the shaft 205.
The technique described with reference to FIG. 4 may be considered a form of sectional magnetic encoding, as only the regions (sections) or channels 260 and 262 are encoded. Depending on the orientation of the encoded sections on the shaft, this technique may be capable of measuring angle of rotation, rotational speed and forces imposed on the shaft, including bending/twisting/torque forces. But this technique is limited to measuring or detecting these parameters only on individual torque sensitive areas on the shaft, i.e., the encoded regions. When the shaft is sectionally encoded, continuous torque measurement is possible only by mounting a magnetic field sensor on the shaft such that the sensor rotates with the shaft. As the sensor rotates, it continuously measures parameters of interest. But requiring the sensor to rotate with the shaft adds complexity to the system, requiring slip rings or wireless data transmission systems and wireless power supplies or batteries.
Various processes and systems have been used to provide accurate and reliable measuring capabilities for a rotating shaft, some of which have been described above. However continued improvements are needed, especially with respect to larger diameter shafts, and enhancements in operational efficiency are desired. The present invention presents a new and nonobvious technique for sectionally encoding the shaft and a pattern of sectionally encoded regions to measure forces imposed on the shaft, especially large diameter shafts. The pattern of encoded regions may also permit simultaneously determining a rotational angle and a rotational speed of the shaft.